Try to think like the crew in terms of how the rounds would be stored for best access and use with the gun and go with that. I doubt there's room for very much in the way of onboardr/ready round storage judging by your last pic and the view of the interior space. Remember the round storage would also have to be done in such a way that it wouldn't interfere with the ability of the crew to also man the gun in action.

here is a link with some photos of the inside of the fighting compartment but they are very bad photos so I don't know how much help they will be..

http://www3.plala.or.jp/takihome/newfolder/Ho-Ni3.html 

Delbert: Thanks for the photos, funny in my web searches none of those came up. Every little bit helps.

Bill: Thanks, you're right, I think I'll go have to go with throwing something together that just makes common sense.

A better photo of the interior hull. I've heard this thing listed as having a crew of five, Second World War Japanese may have been small guys but I can't imagine how three guys could fit in the hull (assuming the commander and gunner stayed in the fighting compartment above) along with the ammo storage as well.

I assume it would be an enclosed horizontal bin here, unless it would be stored in vertical racks running down the left wall. If in the front I'd think the loader could stand behind the bin to hand the ammo up to the fighting compartment. Where the fifth crewman would be I have no idea unless someone misstated the number of crew.